The present invention relates to a toner that is used for developing an electrostatic latent image formed by an electrophotographic process or an electrostatic recording process, and a process for producing the same.
A method for visualizing image information through an electrostatic image, such as an electrophotographic process, is being applied to various fields of art. In the electrophotographic process, an electrostatic image is formed on a photoreceptor by charging and exposing steps and then developed with a developer containing a toner, followed by visualizing an image through transferring and fixing steps. The developer used herein includes a two-component developer formed with a toner and a carrier, and a one component toner using a magnetic toner or a non-magnetic toner solely. As a production process of a toner used in these developers, a kneading and pulverization method is employed, in which a thermoplastic resin is melted and kneaded with a pigment, a charge controlling agent and a releasing agents such as a wax, and after cooling, the mixture is finely pulverized and then classified. Inorganic fine particles or organic fine particles are added depending on necessity to the surface of the particles of the toner for improving the fluidity and tho cleaning property.
The toner shape and the surface structure of the toner formed by the ordinary kneading and pulverization method are in an irregular form and are subtly changed depending on the pulverization property of the materials used and the conditions in the pulverization step. Therefore, it is generally difficult to control to the desired toner shape and the desired surface structure. Particularly, in the case where the toner is formed with a material of high pulverization property, it often occurs that a toner in a further fine particle form is formed, or the toner shape is changed. Under the influence of these phenomena, the developing property of the toner is lowered due to the change in toner shape, for example, the fine particles stick on the surface of the carrier to accelerate deterioration of charge in the case of the one component toner, and the particle size distribution is broadened to cause scattering of the toner, and as a result, deterioration in image quality is liable to occur. In the case where the toner is formed by internally adding a releasing agent, such as wax, while depending on the combination with a thermoplastic resin, the releasing agent is exposed on the surface of the toner to adversely affect Particularly, in the case of the combination of a resin that is somewhat difficult to be pulverized by imparting elasticity with a high molecular weight component and brittle wax, such as polyethylene, exposure of the polyethylene on the surface of the toner is often observed. Although this is advantageous in releasing property on fixing and cleaning of a non-transferred toner from a photoreceptor, the polyethylene on the surface layer is easily transferred to various members by a mechanical force, contamination of a developer roll, a photoreceptor and a carrier is liable to occur, which causes deterioration in reliability.
Furthermore, because the toner shape is irregular, the fluidity cannot be sufficiently established even by adding a fluidizing aid, and the fine particles on the surface of the toner migrate to concave parts of the toner by a mechanical force on use to lower the fluidity with the lapse of time. The developing property, the transferring property and the cleaning property are deteriorated by burying the fluidizing aid into the interior of the toner. Moreover, deterioration in image quality is liable to occur when the toner recovered by cleaning is returned to a developing device and reused. When the amount of the fluidizing aid is increased to prevent the problems, formation of black spots on the photoreceptor and scattering of the particles of the aid occur.
In recent years, a process for producing a toner by an emulsion polymerization aggregation process is proposed in JP-A-63-282749 and JP-A-6-250439 as a method capable of controlling the toner to have a desired toner shape and a desired surface structure. In the process, generally, a resin dispersion is produced, for example, by emulsion polymerization, and is colorant dispersion is separately produced by dispersing a colorant in a solvent, both of which are mixed to form aggregated particles corresponding to a toner particle diameter, followed by fusing by heating to produce a toner. In the method, however, since the surface and the interior of the toner have the same composition, it is difficult to intentionally control the composition of the surface of the toner.
As described in the foregoing, in order to maintain stable performance of the toner under various kinds of mechanical stress in the electrophotographic process, it is necessary to increase the surface hardness or further increase the smoothness of the surface. In order to exert the performance of the releasing agent, it is desirable that it is not exposed to the surface but is present in the vicinity of the surface upon fixing.
There are various problems with respect to the particle size distribution of the toner in the electrophotographic process. The problems include the breakage of the toner by the mechanical force aforementioned, and when the particle size distribution of the toner itself is broad, it influences on the particle size selectivity upon developing, generation of scattering in transferring and easiness in cleaning.
When the particle size distribution is broad in the case of the one-component developer, contamination of a developer roll, a charging roll and a charging blade is liable to occur, and the influence of the fine particle side tends to cause a problem.
Furthermore, a toner having a broad particle size distribution is poor in reliability in a system where a recovered toner by cleaning is reused.
The volume average or number average GSD has been conventionally used as an index for the particle size distribution. With respect to the index, it has been found that an index, GSDpS=D50p/D16p, indicating an extent of tailing on the small diameter side in the number average GSD is important.
The invention has been made to solve the problems associated with the conventional toners and to provide a toner for developing an electrostatic image, a process for producing the same, a developer for developing an electrostatic image and a process for forming an image.
In other words, the invention is
(1) to provide excellent developing and transferring properties,
(2) to obtain excellent performance stability and to provide high image quality and high reliability,
(3) to provide a two-component developer of a long service life that is difficult to cause carrier contamination,
(4) to provide a developer of a small consuming amount of a toner owing to a high transferring efficiency,
(5) to provide a one-component toner that is difficult to bring about contamination of a developer roll, a charging roll and a charging blade,
(6) to provide a toner or a developer used in a toner recycling system, i.e., a system where a toner recovered from a cleaner is reused, which is a toner or a developer exhibiting high reliability, and
(7) to provide high image quality in a cleanerless system, in., a system having no cleaning mechanism.
According to one aspect, the invention relates to a toner for developing an electrostatic image containing a binder resin and a pigment. The toner particles have a particle size distribution index on a small particle side GSDpS in number distribution of a particle diameter represented by the following equation (I) of about 1.27 or less:
GSDpS=D50p/D16pxe2x80x83xe2x80x83(I) 
wherein D50p represents a particle diameter providing a particle diameter accumulation in number distribution of 50%, and D16p represents a particle diameter providing a particle diameter accumulation in number distribution of 16% from a small diameter side,.
According to another aspect, the invention provides a process for producing a toner containing a step of forming a first resin particle dispersion containing binder resin particles and of forming aggregated particles by aggregating the first particles, and a step of fusing the aggregated particles by heating, so as to form a toner particles, the toner particles having a particle size distribution index on a small particle side GSDpS in number distribution of a particle diameter represented by the following equation (I) of about 1.27 or less:
GSDpS=D50p/D16pxe2x80x83xe2x80x83(I) 
wherein D50p represents a particle diameter providing a particle diameter accumulation in number distribution of 50%, and D16p represents a particle diameter providing a particle diameter accumulation in number distribution of 16% from a small diameter side.
According to still another aspect, the invention provides a fine resin particle dispersion containing fine resin particles having a Zeta potential of xe2x88x9250 mV or less in a dispersion of pH 2.5.
According to a further aspect, the invention provides a releasing agent dispersion having a releasing agent dispersed therein. The releasing agent, particles have a volume average particle diameter in a range of about from 100 to 300 nm, and have a particle size distribution index on a small particle side GSDvS and a particle size distribution index on a large particle side GSDvL in volume distribution of a particle diameter represented by the following equations (III) and (IV), respectively, both of which are about 2.0 or less:
GSDvS=D50v/D16vxe2x80x83xe2x80x83(III) 
wherein D50v represents a particle diameter providing a particle diameter accumulation in volume distribution of 50%, and D16v represents a particle diameter providing a particle diameter accumulation In volume distribution of 16% from a small diameter side
GSDvL=D50v/D84vxe2x80x83xe2x80x83(IV) 
wherein D50v represents a particle diameter providing a particle diameter accumulation in volume distribution of 50%, and D84v represents a particle diameter providing a particle diameter accumulation in volume distribution of 84% from a small diameter side.
According to a still further aspect, the invention provides a developer for developing an electrostatic image containing a toner a carrier, the toner containing a binder resin and a pigment and toner particles having a particle size distribution index on a small particle side GSDpS in number distribution of a particle diameter represented by the following equation (I) of about 1.27 or less:
GSDpS=D50p/D16pxe2x80x83xe2x80x83(I) 
wherein D50p represents a particle diameter providing a particle diameter accumulation in number distribution of 50%, and D16p represents a particle diameter providing a particle diameter accumulation in number distribution of 16% from a small diameter side.
According to a still further aspect, the invention provides a process for forming an image containing a step of forming an electrostatic image on an electrostatic latent image holding member, a step of developing the electrostatic latent image with a developer to form a toner image on a developer holding member, and a step of transferring the toner image to a transfer material, the developer containing a toner particles having a particle size distribution index on a small particle side GSDpS in number distribution of a particle diameter represented by the following equation (I) of about 1.27. or less:
GSDpS=D50p/D16pxe2x80x83xe2x80x83(I) 
wherein D50p represents a particle diameter providing a particle diameter accumulation in number distribution of 50%, and D16p represents a particle diameter providing a particle diameter accumulation in number distribution of 16% from a small diameter side.